An integral part of microelectronic fabrication is the use of photoresists to transfer an image from a mask or reticle to the desired circuit layer. After the desired image transfer has been achieved, an etching process is used to form the desired structures. The most common structures formed in this way are metal lines and vias. The metal lines are used to form electrical connections between various parts of the integrated circuit that lie in the same fabrication layer. The vias are holes that are etched through dielectric layers and later filled with a conductive metal. These are used to make electrical connections between different vertical layers of the integrated circuit. A halogen containing gas is generally used in the processes used for forming metal lines and vias.
After the etching process has been completed, the bulk of the photoresist may be removed by either a chemical stripper solution or by an oxygen plasma ashing process. The problem is that these etching processes produce highly insoluble metal-containing residues that may not be removed by common chemical stripper solutions. Also, during an ashing process the metal-containing residues are oxidized and made even more difficult to remove, particularly in the case of aluminum-based integrated circuits. See, “Managing Etch and Implant Residue,” Semiconductor International, August 1997, pages 56-63.
An example of such an etching process is the patterning of metal lines on an integrated circuit. In this process, a photoresist coating is applied over a metal film then imaged through a mask or reticle to selectively expose a pattern in the photoresist coating. The coating is developed to remove either exposed or unexposed photoresist, depending on the tone of the photoresist used, and produce a photoresist on the metal pattern. The remaining photoresist is usually hard-baked at high temperature to remove solvents and optionally to cross-link the polymer matrix. The actual metal etching step is then performed. This etching step removes metal not covered by photoresist through the action of a gaseous plasma. Removal of such metal transfers the pattern from the photoresist layer to the metal layer. The remaining photoresist is then removed (“stripped”) with an organic stripper solution or with an oxygen plasma ashing procedure. The ashing procedure is often followed by a rinsing step that uses a liquid organic stripper solution. However, the stripper solutions currently available, usually alkaline stripper solutions, leave insoluble metal oxides and other metal-containing residues on the integrated circuit.
Another example of such an etching process is the patterning of vias (interconnect holes) on an integrated circuit. In this process, a photoresist coating is applied over a dielectric film then imaged through a mask or reticle to selectively expose a pattern in the photoresist coating. The coating is developed to remove either exposed or unexposed photoresist, depending on the tone of the photoresist used, and produce a photoresist on the metal pattern. The remaining photoresist is usually hard-baked at high temperature to remove solvents and optionally to cross-link the polymer matrix. The actual dielectric etching step is then performed. This etching step removes dielectric not covered by photoresist through the action of a gaseous plasma. Removal of such dielectric transfers the pattern from the photoresist layer to the dielectric layer. The remaining photoresist is then removed (“stripped”) with an organic stripper solution or with an oxygen plasma ashing procedure. Typically, the dielectric is etched to a point where the underlying metal layer is exposed. A titanium or titanium nitride anti-reflective or diffusion barrier layer is typically present at the metal/dielectric boundary. This boundary layer is usually etched through to expose the underlying metal. It has been found that the action of etching through the titanium or titanium nitride layer causes titanium to be incorporated into the etching residues formed inside of the via. Oxygen plasma ashing oxidizes these via residues making them more difficult to remove. A titanium residue removal enhancing agent must therefore be added to the stripper solution to enable the cleaning of these residues. See “Removal of Titanium Oxide Grown on Titanium Nitride and Reduction of Via Contact Resistance Using a Modern Plasma Asher”, Mat. Res. Soc. Symp. Proc., Vol. 495, 1998, pages 345-352. The ashing procedure is often followed by a rinsing step that uses a liquid organic stripper solution. However, the stripper solutions currently available, usually alkaline stripper solutions, leave insoluble metal oxides and other metal-containing residues on the integrated circuit. There are some hydroxylamine-based strippers and post-ash residue removers on the market that have a high organic solvent content, but they are not as effective on other residues found in vias or on metal-lines. They also require a high temperature (typically 65° C. or higher) in order to clean the residues from the vias and metal-lines.
The use of alkaline strippers on microcircuit containing metal films has not always produced quality circuits, particularly when used with metal films containing aluminum or various combinations or alloys of active metals such as aluminum or titanium with more electropositive metals such as copper or tungsten. Various types of metal corrosion, such as corrosion whiskers, galvanic corrosion, pitting, notching of metal lines, have been observed due, at least in part, to reaction of the metals with alkaline strippers. Further it has been shown, by Lee et al., Proc. Interface '89, pp. 137-149, that very little corrosive action takes place until the water rinsing step that is required to remove the organic stripper from the wafer. The corrosion is evidently a result of contacting the metals with the strongly alkaline aqueous solution that is present during rinsing. Aluminum metal is known to corrode rapidly under such conditions, Ambat et al., Corrosion Science, Vol. 33 (5), p. 684. 1992.
Prior methods used to avoid this corrosion problem employed intermediate rinses with non-alkaline organic solvents such as isopropyl alcohol. However, such methods are expensive and have unwanted safety, chemical hygiene, and environmental consequences.
In U.S. Pat. No. 6,465,403 there is disclosed aqueous alkaline compositions useful in the microelectronics industry for stripping or cleaning semiconductor wafer substrates by removing photoresist residues and other unwanted contaminants. The aqueous compositions typically contain (a) one or more metal ion-free bases at sufficient amounts to produce a pH of about 10-13; (b) about 0.01% to about 5% by weight (expressed as % SiO2) of a water-soluble metal ion-free silicate; (c) about 0.01% to about 10% by weight of one or more metal chelating agents and (d) optionally other ingredients.
However, none of the compositions disclosed in the prior art effectively remove all organic contamination and metal-containing residues remaining after a typical etching process. Silicon containing residues are particularly difficult to remove using these formulations. There is, therefore, a need for stripping compositions that clean semiconductor wafer substrates by removing inorganic and organic contamination from such substrates without damaging the integrated circuits. With the widespread use of single wafer tools, there is also a need for formulations that are able to remove metallic and organic contamination in less time and at lower temperatures than compositions in the prior art. Such compositions must not corrode the metal features that partially comprise the integrated circuit and should avoid the expense and adverse consequences caused by intermediate rinses. Tungsten and aluminum lines are particularly susceptible to corrosion upon cleaning with the formulations discussed in the previous paragraph.